(1). Field of the Invention
This application is directed to laminating films in the solid state onto metal substrates. In particular, applying bi-axially oriented polyester films onto metal substrates to create a commercial bond on an industrial processing line whereby multiple desirable commercial properties are simultaneously developed.
(2). Description of Related Art
Others have described laboratory processing steps related to putting films onto metal surfaces. For example, U.S. Pat. No. 5,330,605 describes preheating a metal strip and then laminating a biaxially oriented copolyester resin film. However, a post treating step has been found to be necessary for permanent commercial adhesion in many important markets, and the post treating step is troublesome when used with an oriented polyester film because it can alter crystalline properties. It is difficult to obtain sufficient bonding for demanding stamping applications with additionally demanding chemical resistance requirements. Since crystallinity provides important commercial pencil hardness, toughness, and chemical resistance properties, a high temperature post heating step will change the crystallinity in actual use.
U.S. Pat. No. 5,149,389 and U.S. Pat. No. 5,093,208 describes a thermal laminating process where a metal strip is preheated, laminated, post heated, and quenched in water. The process targets the creation of non-crystalline polyester coating that is generally useful for can making. Unfortunately, the lack of crystallinity is a distinct disadvantage in creating desirable commercial characteristics such as pencil hardness, chemical resistance, and toughness in bending (i.e. coating continuity).
U.S. Pat. No. 5,318,648 describes a thermal laminating process where the cooling process is specifically performed to avoid creating crystallinity in the laminate film. This has similar problems with pencil hardness and toughness properties just described.
U.S. Pat. No. 3,679,513 describes a thermal laminating process for a polyethylene. The process does not describe pretreating the metal surface by raising the surface energy nor does it describe methods of creating crystallinity in the finished laminate film to develop pencil hardness or bending toughness. Polyethylene is not known to develop desirable commercial properties and the low melting point of polyethylene is undesirable for many markets when compared to other polymers.
U.S. Pat. No. 5,679,200 describes a thermal laminating process for applying a film to a metal strip where the laminating rolls provide a specific force. The patent is directed toward a specific laminating nip force related to avoiding the pickup of film onto the nip rolls. The process does not describe pretreating the metal surface by raising the surface energy nor does it describe methods of creating crystallinity in the finished laminate film.
U.S. Pat. No. 5,695,579 describes a thermal laminating process where the polymer coated metal is rapidly and immediately quenched after post treating to ensure that the coating is amorphous. The described process is designed to avoid creating crystallinity in the finished laminate film. The process does not describe pretreating the metal surface by raising the surface energy nor does it describe methods of creating crystallinity in the finished laminate film.
Others have worked on important commercial-technical issues such as the eliminating entrapped air between the film and metal substrate. For example, U.S. Pat. No. 6,200,409 describes an improved laminating process which works on eliminating air bubbles by heating the laminating nip rolls and preheating the film prior to laminating. Similarly, U.S. Pat. No. 6,164,358 describes efforts at reducing air entrapment by using a support roll with a projected film angle. In the later disclosure, a commercially acceptable amount is defined as an 8% area covered by entrapped air. Others, such as U.S. Pat. No. 5,679,200, have attempted to handle trapped air through increased nip forces.
Important commercial markets are open to lamination provided that acceptable adhesion, pencil hardness, bending toughness, and corrosion protection can be simultaneously achieved. These markets are currently served by the pre-painted coil coated industry.
Typical products include the following:    1) Building and Construction Products such as: Roofing & Siding, Exterior Accessories, Structural & Mechanical, Interior Components, Manufactured Housing, Garage Doors, and Doors & Windows.    2) Transportation Products such as: Passenger Cars, Vans, & Light Trucks, Trucks & Semi-Trailers, Buses, and Travel Trailers & Recreational Vehicles.    3) Business and Consumer Products such as: Large & Small Appliances, Electronics, Water Heaters & Water Softeners, Heating & Cooling Equipment, Home & Office Furniture, Window Equipment, Toys & Sporting Goods, Fixtures & Shelving, and Lighting.    4) Containers and Packaging Products such as: Cans, Ends, Tabs, Crowns, & Closures, Barrels, Drums, & Pails, Strapping & Seals, and Draw & Ironed can bodies,    5) Other/Miscellaneous Products such as: Machinery & Industrial Equipment, Electrical Equipment, and Signs & Displays.
It is important to note that the referenced patents have not resulted in a commercially viable high production thermal laminating line in the United States. The difficulties in simultaneously scaling up production, creating an economically viable process, and developing suitable commercial properties have been strong barriers to the actual implementation of a laminating process. The previous efforts by others have been lacking in important technical aspects of cooperation between the processing steps, economic viability, and suitable commercial properties.
Current high production laminating methods in the United States address metal substrates, i.e. 0.005″ and above, are primarily directed at utilizing press on adhesives which are applied by a roller onto the metal substrate, and the adhesive is dried in an oven prior to the laminating step. This process is commonly added to, or is a part of, a commercial coil paint line. The application of the film to the metal substrate is generally done close to ambient temperatures. The adhesive is separately applied to the metal substrate and is usually not a part of the film, such as a multilayer film.
It is important that high production thermal laminating methods have little or no entrapped air between the metal substrate and the film. Entrapped air causes thinning of the coating at an unpredictable amount. In particular, when a formed part is bent and the bend occurs where an air bubble exists in the coating, an increased likelihood of failure results. Air entrapment is a serious issue when the air bubble size is significant relative to the coating thickness, and the frequency is high. It is also visually disturbing at an 8 percent level to a customer, on a surface area basis, and raises unnecessary questions about process control.
It is important that the coating has the necessary pencil hardness, that is, surface scratch resistance, and also suitable bending capability. This will allow normal material handling without scratches. Coating hardness must be balanced against brittleness. A hard coating has an increased likelihood of splitting on the bend of a formed part. If the coating splits, the metal is exposed and there is likelihood of a corrosive failure at that spot.
From a commercial standpoint, it is important that the coating also has suitable chemical resistance after forming a finished part, which comes from a stamping or bending operation. In the can-making industry, pack tests are performed that are very demanding on forming and chemical resistance. Formed parts, such as a can, are packed with typical or harsh commercial materials and stored at an elevated temperature to accelerate any corrosive action. Test storage temperatures are 100 to 120° F. for periods of one to twenty four months. More commonly, a one, three, or six month test is sufficient to determine if a coating will fail in a pack test, depending upon the product. The main success/failure criterion is whether there is defect free through a can making (i.e. stamping) operation, visual corrosion, and whether the coating delaminates.
In summary, it has been difficult to develop the necessary simultaneous properties for a commercial thermoplastic coating on a metal substrate at an economical cost. The coating needs the simultaneous capability of: developing suitable bonding to the metal substrate, economical production, having suitable pencil hardness, eliminating air entrapment, obtaining a high level of chemical resistance, and having the ability to withstand part forming without splitting.